The signal generation circuitry for mobile radios is a complex process with component cost and component count issues being high priorities for consideration in the radio design. A typical circuit for generating radio frequencies in a mobile radio, and in particular for generating frequency modulated radio frequencies, involves the use of a voltage controlled oscillator circuit (VCO). In generating a modulated transmit radio frequency, the VCO uses a voltage supply to control the frequency of operation and a is supplied (modulated) with the information e.g. voice and/or data, to be transmitted. In designing such VCO circuits, there is a design trade-off between VCO settling time--the time it takes for a particular frequency to be generated and remain within a specific frequency tolerance--and the frequency bandwidth of the audio signal input to the VCO circuit.
In order to resolve this trade-off dilemma, a typical approach is to use an additional low-frequency reference modulation input to the VCO circuit. The low-frequency reference modulation input is applied during a radio tune-up process to compensate for any low-frequency attenuation of the information signal. Thus, the settling time of the VCO circuit can be optimized by the radio designer at the expense of any low-frequency performance of the VCO as any poor attenuation performance can be compensated for. A typical approach to align the attenuation of the reference modulation input for compensation purposes is to inject a 10 Hz square wave signal into the transmitter and then to manually tune the VCO loop filter response to provide a 10 Hz square wave response out of the VCO circuit. The 10 Hz square wave input comprises a multiple series of 10 Hz tones, some of the lower frequencies of which will be attenuated more than the higher frequencies and hence will be compensated for by the VCO manual tuning.
A disadvantage associated with this approach is that manual tuning is time consuming and the tune-up process requires specific test equipment, e.g. square wave generator, oscilloscope, system analyzer. In addition, the manual tuning process for obtaining a square wave output is obviously subjective and therefore has some inherent inaccuracy associated with it.
This invention seeks to provide a transmitter circuit, and method of operation, to mitigate some of the problems highlighted above associated with optimizing frequency modulation circuits.